Semi-suspended coplanar waveguide on a printed circuit board

ABSTRACT

A printed circuit board includes two differential signal traces, a layer of core material, a layer of filler material, and a ground plane. The filler material is replaced by an air core under the differential signal traces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional ApplicationNo. 11/148,564, filed Jun. 9, 2005, which is a continuation of U.S.Non-Provisional Application No. 10/354,068, filed Jan. 30, 2003, nowU.S. Patent No. 6,924,712, issued Aug. 2, 2005, all of which areincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to printed circuit boards and moreparticularly, to printed circuit boards (PCBs) with coplanar waveguidesand high frequency applications.

2. Related Art

Modern IC devices operate at increasingly higher frequencies. Asfrequencies of circuits placed on PCBs reach tens of gigahertz, theelectrical characteristics of PCB traces resemble high-speed signaltransmission lines, rather than DC electrical circuits. The higherfrequencies and resultant shorter signal rise times expose PCBperformance limitations that are manifested by signal integrityphenomena such as ringing, dielectric losses, reflections, groundbounce, and cross-talk.

Stripline, or microstrip transmission lines, are commonly used as ameans of transmitting signals from one portion of the printed circuitboard to another. Typically the impedance of the waveguide formed by thestripline or the microstrip structure is matched to 50 ohms.

The microstrip transmission line is a strip conductor that is separatedfrom a ground conductor by a dielectric substrate. However, a problemwith the microstrip line is that it has a high transmission loss at highfrequencies.

Conventional art has attempted to deal with the problem of losses in thedielectric by turning to exotic materials with relative dielectricconstant ε_(r) of down to approximately 2. However, these materials aretypically very expensive, highly flammable, and exhibit poor peelcharacteristics. The capacitive effects that are created by the presenceof the dielectrics, even low ε_(r) dielectrics, add to system losses,and degrade signal integrity.

As a result, there is a need for structures capable of transmitting highfrequency signals, which minimize transmission losses, are of smallsize, allow for easy and inexpensive fabrication and integration, andstill enable desired performance requirements to be met.

SUMMARY OF THE INVENTION

The present invention is directed to a semi-suspended coplanar waveguideon a PCB and a method of its manufacture that substantially obviates oneor more of the problems and disadvantages of the related art.

There is provided a printed circuit board including two differentialsignal traces, a layer of core material, a layer of filler material, anda ground plane. The filler material is replaced by an air core under thedifferential signal traces.

In another aspect there is provided a method of forming a printedcircuit board including forming a stack of layers of filler material andcore material over a conductive layer. A channel is formed in the fillermaterial and the core material. A core layer is formed over the channeland over remaining portions of the filler material and the core layer.Signal traces are formed over the channel so as to form an air corewaveguide.

In another aspect there is provided a method of forming a printedcircuit board including forming a layer filler material over aconductive layer, forming a channel in the filler material, forming acore layer over the channel and over remaining portions of the fillermaterial, and forming signal traces over the channel so as to form anair core coplanar waveguide.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to illustrate exemplaryembodiments of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 shows a cross-sectional view of a semi-suspended coplanarwaveguide of one embodiment of the invention.

FIG. 2 shows an isometric view of a coplanar waveguide formed on aprinted circuit board using the invention.

FIG. 3 shows an exemplary diagram of the coplanar waveguide of FIG. 2.

FIG. 4 shows a photograph of a cross-section of a printed circuit boardformed according to one embodiment of the present invention.

FIG. 5 shows three photographs of a plan view of a printed circuitboard, with successive layers being peeled off.

FIGS. 6A and 6B illustrate performance obtained using conventionalprinted circuit boards with 8 mil cores.

FIGS. 6C, 6D and 6E illustrate improvement in performance obtained usingthe present invention.

FIGS. 7-8 illustrate alternative PCB structures that correspond to FIG.6D-6E.

FIG. 9 shows the method of an embodiment of the present invention inflow chart form.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 shows a cross sectional view of the semi-suspended coplanarwaveguide of one embodiment of the present invention. As shown in FIG.1, a PCB 100 includes a suspended differential trace 101 and a suspendeddifferential trace 102. The two differential traces 101, 102 may be madeof conductive material, for example, copper or aluminum. The traces alsomay be gold plated copper. The use of copper is generally preferred.

On the left and right sides of the differential traces 101, 102 are aground trace 103 and a ground trace 104, respectively. A 2 mil corelayer 105 is positioned under the traces 101, 102 and the ground traces103, 104. The core material may be FR4, or any number of conventionalmaterials used in PCB manufacturing.

A 2 mil prepreg layer 106 is positioned under the core layer 105. Theprepreg layer 106 is a filler layer, for example, a fiberglass layer. A2 mil core layer 107 is stacked below the prepreg layer 106. Anotherprepreg layer 108 is stacked under the core layer 107. A copperreference plane 109 is at the bottom of the PCB 100. An air core 110 islocated below the core layer 105.

Epoxy may be used to attach core material to the copper backing or thetraces 101, 102. Epoxy may also be used to attach core material to theprepreg material. It will be appreciated that FIG. 1 is not drawn toscale, and actual thickness of the epoxy “layer” is much smaller thanindicated in the figure.

FIG. 2 shows a method of calculating losses in the transmission lineformed by the structure shown in FIG. 1. That is, FIG. 2 provides arepresentation of the PCB 100 for use in deriving equations related tothe transmission characteristics of the transmission line depicted inFIG. 1. Specifically, the PCB is shown in FIG. 2 oriented with respectto a three-dimensional axis 202. Further, a trace thickness 204, a tracewidth 206, and a trace depth 208 of the PCB are shown. The PCB 100, asdepicted in FIG. 2, includes a copper trace 210, a FR4 core layer 212,an air dielectric 214, and a ground plane 216. FIG. 3 depicts anequivalent circuit diagram 300 of the PCB shown in FIG. 2. Theequivalent circuit diagram 300 includes a surface resistance 302, a lineinductance 304, a conductance 306, and a capacitance 308. By using thedepiction of the PCB as shown in FIG. 2 along with the equivalentcircuit diagram 300 as shown in FIG. 3, the surface resistance of theline is defined as$R_{s} = \frac{1}{W \cdot \sigma_{cond} \cdot \delta}$and the capacitance is defined as,$C = {\frac{Q}{V} = {\frac{\int{D \cdot {\mathbb{d}S}}}{V} = {\frac{ɛ_{r}{\int{\int{E_{x}{\mathbb{d}S}}}}}{\int{E_{x}{\mathbb{d}l_{x}}}} = {\frac{ɛ_{r} \cdot E_{x} \cdot W}{E_{x} \cdot d} = {\frac{ɛ_{r} \cdot W}{d}.}}}}}$

In the above derivation ε_(r) is the dielectric constant of the corewhich in this new architecture is approximately equal to 1. The lineinductance is defined as$L = {\frac{\int{\int{B{\mathbb{d}S}}}}{I} = {\frac{\int{\int{{\mu \cdot H_{y}}{\mathbb{d}S}}}}{I} = {\frac{{\mu \cdot H_{y}}d}{I} = {\frac{\mu \cdot d}{W}.}}}}$

The conductance can be defined as,$G = {\frac{\int{\int{J \cdot {\mathbb{d}S}}}}{V} = {\frac{\sigma_{diel} \cdot {\int{\int{E_{x} \cdot {\mathbb{d}S}}}}}{\int{E_{x} \cdot {\mathbb{d}l_{x}}}} = {\frac{\sigma_{diel} \cdot E_{x} \cdot W}{E_{x} \cdot d} = \frac{\sigma_{diel} \cdot W}{d}}}}$where, in the above equations, C is capacitance, Q is charge, V isvoltage, D is surface charge density, I is current, σ is conductivity, εis dielectric constant, dS is differential surface element, W is tracewidth, l is trace length, d is trace thickness, B is magnetic density, Jis current density, μ is magnetic permeability, H is magnetic intensity,δ is the skin depth, or depth of penetration, of the transmission line,H_(y) is the magnetic intensity with respect to the y-axis, E_(x) is theelectric field with respect to the x-axis, and dl_(x) is differentiallength element relative to the x-axis.

FIG. 4 is a photograph of a cross section of the printed circuit boardof the present invention that utilizes the air gap 110 under the 2 milcore layer 105 (not shown due to the relative dimensions of the variouselements shown in the photograph) and the differential traces 101, 102.The “boxed” portion of the photograph corresponds to FIG. 1.

FIG. 5 shows a series of photographs of the printed circuit board asviewed from the top, with successive layers of the core material andprepreg material being peeled off, moving from left to right in thefigure.

FIGS. 6A-6E illustrate the improvement obtained by the use of the aircore (cavity) in a semi-suspended coplanar waveguide. FIGS. 6A-6B showperformance obtained using conventional printed circuit boards with 8mil cores. As may be seen, the jitter is approximately 14-16 picoseconds(ps), the rise times are on order of 35 picoseconds, and the fall timesare on the order of 32-33 picoseconds. Specifically, FIG. 6A depictsfall time as the time separation between a trace point 602 and a tracepoint 606. The time separation between the trace point 602 and the tracepoint 606 is represented by a fall time indicator 612 as 32.4 ps. FIG.6A depicts rise time as the time separation between a trace point 604and a trace point 608. The time separation between the trace point 604and the trace point 608 is represented by a rise time indicator 610 as35.1 ps. FIG. 6A depicts jitter as a time overlap 614 at the crossingpoint of the signal trace between the trace points 604 and 608 and thesignal trace between the trace points 602 and 606. The jitterrepresented by the time overlap 614 is 14.9 ps.

Similarly, FIG. 6B depicts fall time as the time separation between atrace point 616 and a trace point 620. The time separation between thetrace point 616 and the trace point 620 is represented by a fall timeindicator 626 as 32.9 ps. FIG. 6B depicts rise time as the timeseparation between a trace point 618 and a trace point 622. The timeseparation between the trace point 618 and the trace point 622 isrepresented by a rise time indicator 624 as 35.6 ps. FIG. 6B depictsjitter as a time overlap 628 at the crossing point of the signal tracebetween the trace points 618 and 622 and the signal trace between thetrace points 616 and 620. The jitter represented by the time overlap 614is 16.0 ps.

FIGS. 6C-6D show the improvement in performance using the air core ofthe present invention. As may be seen from FIG. 6C (which has a 2 milcore 105 and 6 mil air gap 110), jitter (depicted as a time overlap 642at the crossing point of the signal trace between a trace point 632 anda trace point 636 and the signal trace between a trace point 630 and atrace point 634) is reduced down to 12.1 picoseconds, rise time(depicted as the time separation between the trace point 632 and thetrace point 636 and represented by a rise time indicator 638 as 24.9 ps)is reduced to under 25 picoseconds, and fall time (depicted as the timeseparation between the trace point 630 and the trace point 634 andrepresented by a fall time indicator 640) is reduced down to 20.4picoseconds, a roughly 30% improvement. Note also that the quality ofthe eye is substantially improved, with sharper edges, and less ripple.

A similar performance, as shown in FIG. 6D, is obtained with the use ofthe PCB as shown in FIG. 7 that has a 4 mil core 105 and a 4 mil aircore 110. Specifically, FIG. 6D depicts fall time as the time separationbetween a trace point 644 and a trace point 648. The time separationbetween the trace point 644 and the trace point 648 is represented by afall time indicator 654 as 23.6 ps. FIG. 6D depicts rise time as thetime separation between a trace point 646 and a trace point 650. Due tofalling edge triggering, the rise time displayed on the oscilloscopeoutput is incorrect. However, as shown in FIG. 6D, the rise time isapproximately equal to the fall time. Specifically, the time separationbetween the trace point 646 and the trace point 650 is represented by arise time indicator 652 as approximately 24 ps. FIG. 6D depicts jitteras a time overlap 656 at the crossing point of the signal trace betweenthe trace points 646 and 650 and the signal trace between the tracepoints 644 and 648. The jitter represented by the time overlap 656 is14.4 ps. FIG. 7 shows the PCB as having a suspended differential trace101 and a suspended differential trace 102. PCB 100, as depicted in FIG.7, also has a ground trace 103 and a ground trace 104. FIG. 7 also showsa 1 mil prepreg layer 106, a 2 mil core layer 107, a 1 mil prepreg layer108, and a copper reference plane 109.

FIG. 6E shows performance of an alternative air core structure where thePCB has an 8 mil core with 3 mil drilled cavity 105 and a 4 mil air core110, as illustrated in FIG. 8, where performance comparable to thatshown in FIG. 6D is obtained. Specifically, FIG. 6E depicts fall time asthe time separation between a trace point 658 and a trace point 662. Thetime separation between the trace point 658 and the trace point 662 isrepresented by a fall time indicator 666 as 21.3 ps. FIG. 6E depictsrise time as the time separation between a trace point 660 and a tracepoint 664. Due to falling edge triggering, the rise time displayed onthe oscilloscope output is incorrect. However, as shown in FIG. 6E, therise time is approximately equal to the fall time. Specifically, thetime separation between the trace point 660 and the trace point 664 isrepresented by a rise time indicator 668 as approximately 21 ps. FIG. 6Edepicts jitter as a time overlap 670 at the crossing point of the signaltrace between the trace points 660 and 664 and the signal trace betweenthe trace points 658 and 662. The jitter represented by the time overlap670 is 12.4 ps.

FIG. 8 depicts the PCB as having as having a suspended differentialtrace 101 and a suspended differential trace 102. The PCB 100, asdepicted in FIG. 8, also has a ground trace 103 and a ground trace 104.FIG. 8 also shows a 1 mil prepreg layer 106 and a copper reference plane109. Note that generally, the larger air core 110 provides improvement,however, overall configuration needs to be optimized to provide 50 ohmimpedance, so as to avoid reflections and other distortions that occurduring high frequency operation. Also, trace dimensions (e.g., width)needs to be optimized as well, to provide matched 50 ohm impedance

The preferred method of manufacturing the semi-suspended coplanarwaveguide of the present invention is through the use of drilling of thechannel. Thus, with reference to FIG. 1, layers 109, 108, 107 and 106may be stacked on top of each other and epoxied together, forming alower portion of the PCB 100. A numerically controlled drill may then beused to form a channel for the air core 110. The core layer 105 is thenplaced on top of the air core 110 and the prepreg layer 106. A copperlayer is then formed on top of the core layer 105, and the differentialsignal traces 101, 102, and the ground traces 103, 104 are patterned inthe conventional manner. The differential signal traces 101 and 102, aswell as the ground traces 103 and 104, can be etched using photomasking.

As an alternative, the air core 110 may be formed using photomasking andetching techniques, although that approach is believed to beconsiderably more expensive.

FIG. 9 shows an embodiment of a method of the present invention in flowchart form. A stack of layers of filler material and core material overa conductive layer are formed (step 902). A channel is formed in thefiller material and the core material (step 904). A core layer is formedover the channel and over remaining portions of the filler material andthe core material (step 906). Signal traces are formed over the channeland over the core material so as to form an air core coplanar waveguide(step 908). The differential signal traces are bonded to the core layerusing an epoxy (step 910).

It will also be appreciated that the dimensions specified above areexemplary only, and do not limit the invention described herein.

The present invention results in a transmission structure on a printedcircuit board that provides for higher bandwidth, higher signalintegrity, less jitter, and lower loss than conventional printed circuitboards. It may be used in such applications as backplanes, transceiverstructures, serializer/deserializer structures, etc., particularly wherehigh frequency application (over 1 GHz, and particularly tens of GHz) isrequired. The present invention also provides an advantage in that itavoids the use of expensive and difficult to work with materials, andutilizes only commercially relatively inexpensive manufacturingtechniques.

It will be understood by those skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A method of forming a coplanar waveguide having an air corecomprising: forming a layer of filler material over a conductive layer,the layer of filler material comprising at least one prepreg layer andat least one core material layer; forming a channel in the fillermaterial; forming a core layer over the channel and over remainingportions of the filler material; and forming two differential signaltraces over the channel and over the core layer.